Field of the Invention
The present invention relates to an image sensor and driving method thereof, and an image capturing apparatus and, more specifically, to an image sensor having pixels capable of receiving light that has transmitted through divided pupil regions of an imaging lens, and driving method thereof, and an image capturing apparatus including the image sensor.
Description of the Related Art
Conventionally, there is a technique for performing image shooting and phase difference focus detection using a structure in which each of pixels that constitute an image sensor receives bundles of light that have transmitted though different pupil regions of an imaging lens. Japanese Patent Laid-Open No. 2001-083407 discloses a structure in which two photodiodes (PD) are provided for each microlens of each pixel so that respective PDs receive bundles of light that have transmitted through different pupil regions of an imaging lens. Then, using signals output from the respective PDs, detection of an image offset amount, namely, phase difference detection, is performed, and a defocus amount is calculated from the image offset amount, thereby performing focus detection. Further, by adding signals of the respective PDs, that have received bundles of light passing through the different pupil regions of an image lens, for each pixel, it is possible to generate an image signal for viewing.
In an image sensor having two PDs for each microlens, in order to read out signals from two PDs independently, it takes twice as long as time required for reading out a signal from a pixel having one PD, if the readout is performed in the same manner. Accordingly, there is suggested a technique for shortening a readout time by reducing the number of signals used for the phase difference detection in consideration of color arrangement of a color filter that covers each pixel of the image sensor. Japanese Patent Laid-Open No. 2014-072541 discloses an image sensor in which pixels for obtaining an image (imaging pixel) and pixels for focus detection (focus detection pixels) are alternately arranged in a checker pattern. More specifically, the image sensor is covered with a Bayer color filter, wherein red filters and blue filters cover the imaging pixels, and green filters cover the focus detection pixels. Since the green filters have spectral transmission characteristics in the middle of spectral transmission characteristics of red and blue filters, this arrangement is considered preferable.
However, it is not guaranteed that colors of a subject include a large amount of green color component, and moreover, color component distribution suitable for focus detection differs between light sources that illuminate the subject. Furthermore, even if a subject includes a large amount of green color component, there is no guarantee that a high frequency component that is suitable for focus detection resides in green color. Thus, it is not always possible to read out focus detection pixels suitable for conditions of the subject when the technique disclosed in Japanese Patent Laid-Open No. 2014-072541 is used.